1. Field of the Invention
This invention relates to chemical vapor deposition (CVD) of epitaxial semi-insulating layers and, more particularly, to deposition of semi-insulating gallium arsenide layers.
2. Art Background
Gallium arsenide epitaxial layers are used in a wide variety of semiconductor devices. Semi-insulating layers of GaAs are often used as buffer layers between a substrate and active semiconductor region of a device. For example, GaAs based field effect transistors (FETs) are manufactured with a semi-insulating layer of epitaxial GaAs between a single crystal substrate and an epitaxial active region of n-type GaAs. This semi-insulating epitaxial layer is grown using conventional techniques such as chemical vapor deposition (CVD). During the deposition process a deep acceptor dopant is introduced into the chemical vapor flow containing an n-type GaAs deposition precursor--a gas phase composition which leads to deposition of n-type GaAs when contacted with a substrate heated to an appropriate temperature. (Deep acceptors are those having energy levels near the midgap energy of the particular semiconductor material.)
These intermediary semi-insulating epitaxial layers serve two important functions. The substrate, even after the initial cleaning process, invariably has residual contamination. If the active layer is deposited directlyon this contaminated surface, its electrical properties are degraded. When a semi-insulating intermediary layer is deposited on the substrate, and the active layer is subsequently deposited on the intermediary layer, this contamination does not occur. The intermediary layer is sufficiently thick to prevent the contamination from diffusing into the active region. Since the active region is deposited on the intermediary layer without removing the substrate from the deposition apparatus, there is no possibility of contamination at the active region/intermediary region interface.
A second advantage in using an intermediary layer is also attained. Active regions grown directly on the substrate usually yield degraded device characteristics. These properties arise from undesirable interface states which occur at the interface with the substrate. By removing the active region from the substrate/epitaxial layer interface, degradation of electrical properties resulting from these states are substantially reduced.
These advantageous semi-insulating layers must, depending on the application, satisfy certain criteria to maintain device properties. For example, for FETs the intermediary layer must be highly resistive to prevent leakage currents in the device ultimately manufactured which result from current flow through this layer. Typically, resistivities greater than 10.sup.3 ohm-cm preferably greater than 10.sup.6 ohm-cm are desired for applications such as FETs. A deep acceptor dopant is introduced into the normally n-type GaAs so that semi-insulating properties are achievable. This dopant must have the properties that (1) sufficient concentrations can be employed so that the desired resistivity is attained, (2) substantial interference with the growth of a sufficiently regular surface does not occur and (3) adaptability to conventional techniques such as CVD is possible.
For many applications such as GaAs FETs, a chromium dopant has been employed to provide sufficient resistivities for intermediary semi-insulating GaAs layers. The layers produced typically have resistivities in the range 10.sup.3 to 10.sup.10 ohm-cm. Despite the advantages of using a chromium containing dopant, a convenient method of introducing this dopant into the GaAs epitaxial intermediary layer has not yet been found. For example, attempts to chromium dope GaAs layers have been made by using a CrO.sub.2 Cl.sub.2 source. In this method, an inert gas or a gas which reduces CrO.sub.2 Cl.sub.2 such as helium or hydrogen respectively is passed through a bubbler containing the CrO.sub.2 Cl.sub.2. The carrier gas with its fraction of CrO.sub.2 Cl.sub.2 is then passed through an entry tube into the deposition apparatus. However, CrO.sub.2 Cl.sub.2 decomposes at the processing temperatures required for the GaAs deposition. Therefore, particulate decomposition fragments form. This particulate matter either adheres onto the walls of the dopant entry tube or enters the reaction chamber. The portion entering the reaction chamber can fall into the growing epitaxial layer and produce unacceptable surface irregularities. Additionally, chromium particulate matter remains in the reaction chamber after growth of the intermediary epitaxial layer is finished. The semiconducting active region is then grown without venting the system to prevent the contamination problems discussed earlier. However, since chromium entities remain in the reaction chamber they can be incorporated in the active region and degrade its semiconducting properties.
Thus, although chromium is a desirable dopant for producing semi-insulating GaAs intermediate layers, it is associated with contamination and undesirable surface features.